1. Field of the Invention
The present invention relates to acoustic resonators and sound chambers.
The present application claims priority on Japanese Patent Application No. 2009-272891 and Japanese Patent Application No. 2010-239875, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, a variety of sound absorbing structures using acoustic resonators has been developed and disclosed in various documents such as Patent Documents 1 and 2.    Patent Document 1: Japanese Patent Application Publication No. H07-302087    Patent Document 2: Japanese Patent Application Publication No. H08-121142
Patent Document 1 discloses a sound absorbing structure aimed at reducing sound pressure in a low frequency range, wherein a plurality of resonance pipes having different lengths, each of which has an opening end and an opposite closed end, adjoins with their opening ends. Patent Document 2 discloses an intake noise reduction device of an internal combustion engine, which shifts a resonance frequency into a low frequency range by way of a resonance chamber communicating with an intake duct via a communicating tube. Patent Document 2 employs a Helmholtz resonator equivalent to a spring-mass resonance system in which an air of the communicating tube serves as a mass component while an air of the resonance chamber serves as a spring component and in which a sound absorbing material is attached to a certain part of the communicating tube. In the Helmholtz resonator, an internal air of the sound absorbing material serves as a mass component, which is increased to cause a resonance frequency to shift into a lower frequency range compared with another resonator precluding a sound absorbing material.
The sound absorbing structure of Patent Document 1 needs to increase the length of a cavity of each resonance pipe as its resonance frequency decreases so as to decrease sound pressure at a low frequency via a resonance phenomenon; hence, each resonance pipe needs to be increased in size. The Helmholtz resonator of Patent Document 2 is formed in a certain shape having sufficient dimensions securing a uniform distribution of sound pressure relative to an incidence direction of sound waves (or a height direction of the Helmholtz resonator) in a resonance chamber. That is, the Helmholtz resonator is designed to fix a constant sound pressure in a resonance chamber. In addition, the resonance chamber needs to increase its volume as its resonance frequency decreases, so that the width dimension of the resonator may become larger than the height dimension of the resonator. This makes it difficult to install the resonator due to interference with peripheral components. In the case of a Helmholtz resonator demonstrating a sound absorption effect at 160 Hz, for example, it needs to increase the overall size such that the diameter is set to 145 mm while the height is set to 130 mm approximately.